This invention pertains to transmitting highly time-compressed audio frequency information over a television video channel.
The format of a television video channel is well known. The NTSC standard in the United States may be taken as a suitable example. The bandwidth of the channel is approximately 4.5 megahertz (MHz). Analog electrical variations produced by scanning a visual scene are produced along each line, which is initiated and terminated by the horizontal blanking interval and carries a horizontal synchronizing pulse. The analog electrical variations occupy one side of the axis of the video waveform and the synchronizing and blanking pulses occupy the other side.
The NTSC standard utilizes 525 lines per frame in scanning over the visual scene. These are separated into two fields of half as many lines, interlaced, so that all of the even numbered lines are scanned in one field and all of the odd numbered lines are scanned in the next field. Each field is executed in 1/60th second, thus one frame is executed in 1/30th second.
Color television is accomplished by adding certain auxiliary pulses on the back porch of the horizontal blanking interval, but this is of no moment in the practice of this invention.
It would be possible to modulate a sequence of sound upon a frame of television waveform, particularly if the line synchronizing and blanking pulses were omitted. However, since the duration of a frame is only 1/30th second, only part of one syllable of speech would be transmitted over the television video channel. Any greater duration would seriously interrupt the television synchronizing process and the attempt would be impractical.